Public Land Mobile radio Network (PLMN) is a generic term for a mobile wireless network that is centrally operated and administrated by an organization and uses land-based radio frequency transmitters or base stations as network hubs. PLMNs can stand alone and interconnect with one another or connect to a fixed system such as the PSTN.
In the near future there will be an increase in traffic load on the packet switched part of the PLMNs, such as GSM/(EDGE) GPRS, UMTS (WCDMA) and CDMA2000 due to the development and implementation of new packet based services. One such packed based service that utilizes packet switched bearers is referred to as Push to talk over Cellular (PoC). Push to talk over Cellular (PoC) is currently being standardized and agreed upon in an industry consortium known as the Open Mobile Alliance (OMA) forum.
Push-to-talk over Cellular (PoC) is being developed for handsets in networks such as GSM/(EDGE) GPRS networks, UMTS (WCDMA), and CDMA systems. PoC is basically a voice chat for cellular telecommunication systems. PoC provides quick one-to-one or group communication, providing something like a short instant VoIP service, which feels like “walkie talkies”.
PoC enabled handsets will most likely be equipped with a PoC-button. The PoC button may (for example) be: a dedicated hardware button; an assigned button on a standard keypad; or, a software button used in e.g. pressure sensitive screens. When the PoC button is pressed, the handset is connected to another user or user group via a PoC server. The first releases of PoC provide half-duplex service, although full duplex may be available at a later stage.
Combinational services enrich the Circuit-Switched (CS) voice service of today, with images, music and video-clips. The images and/or video-clips would utilize the packet switched (PS) part of the PLMNs when being transferred from one user's client to another user's client.
Much effort and investment has been made to develop a fully packet switched solution for voice communication. Such solution is often referred to as Voice over IP (VoIP) since it is assumed that the Internet Protocol (IP) will be used to carry the media. Now this work will be reused to further enhance VoIP. It is anticipated that in the near future it will be possible to offer combinations of, for example, PoC with video (PoC Video Group Message) and/or images, and (telephony like) VoIP with video and/or images, even over current deployed PLMNs. Services that combine voice and image/video (regardless if the voice is packet switched or circuit switched) by the PoC button sometimes go under the name Push to Show (PtS) services.
One problem associated with such services is how to accurate set the media (e.g. audio, video, image) playback/rendering point to optimize the end-to-end (E2E) content delivery performance. This problem may arise in various situations. For example, the delay of the path of transfer may drastically change due to changes of transport-related settings or states in the nodes involved in the transport.
A channel type switch such as that which occurs in certain radio access bearer (RAB) realizations for wideband code division multiple access (WCDMA) is one illustration of this example problem situation for a packet switched audio service, such as VoIP or PoC. WCDMA is described, e.g., in 3GPP, “Technical Specification Group Radio Access Network; Radio Resource Control (RRC), Protocol Specification”, TS 25.331 V4.13.0, March 2004.
Consider FIG. 8, which depicts the Radio Resource Control (RRC) state machine of WCDMA. The RRC state starts up in idle mode. When data is to be transmitted, the RRC state may go to CELL_DCH (camping on a dedicated channel) or to CELL_FACH (camping on a common channel). When the RRC state is in CELL_DCH and the transmitter throughput drops below a certain limit during a certain time period, a channel type down switch to CELL_FACH is executed. After yet some time without any new data the RRC state will switch down further to idle mode. However, if data is received prior to the down switch to idle mode, then depending on the amount of data (e.g., the Radio Link Control (RLC) buffer reaches a certain threshold), the RAB is switched to RRC state CELL_DCH. The problem for the audio is that some media will be transferred during the CELL_FACH state, and when the state switch occurs there will be a delay in the transmission of the media with the result of an annoying gap in the playout of audio to the recipient. Accordingly, during this switching operation, which generally requires about 0.5-1.0 sec, the data traffic is interrupted. For two users connected to each other via a PoC server, switching may occur either in the uplink or in the downlink or both. Consequently, an interruption of the data traffic lasting about 0.5-2.0 sec often occur, which, in turn, entails an interruption of the playout of the talk burst (a talk burst in PoC is one or several sentences spoken from the activating of the PoC button to releasing it), which the receiving part perceives or experiences as very annoying.
Furthermore, interruptions may also be caused by handover. In this case, the interruption(-s) will occur when the playout already has begun thereby entailing an interruption of the playout of the talk burst (a talk burst in PoC is one or several sentences spoken from the activating of PoC button to releasing it), which the receiving part perceives or experiences as very annoying.
There are a number of solutions addressing this problem. A first obvious solution is to increase the buffering time to a few seconds to avoid interruptions in the playout. This solution is however impaired by the drawback that the playout at the receiver(-s) will be delayed thereby increasing the response time, which, in turn, has a negative effect on the interaction among the participants in a VoIP service data transmission, such as a PoC data transmission. A second solution would be to use an selective buffer management in the client. The client increases the buffering time when channel switching is anticipated and otherwise a smaller buffer is used. When channel switching is anticipated this solution uses a longer buffering time, and, accordingly, the same problems as in the first solution will arise. Furthermore, an erroneous prediction whether channel switching will occur or not might lead to buffer underrun at the receiver.
Thus, there is a need for an improved VoIP service for communications systems with regard to perceived interactivity in user communications and, in particular, for a method for handling or compensating for interruptions or delays in the data traffic during VoIP service data transmissions, for example, PoC service data transmissions caused by channel switching.